Showing papers in "Journal of the American Ceramic Society in 1959"

Piezoelectric and Dielectric Properties of Ceramics in the System Potassium—Sodium Niobate

Abstract: High radial coupling coefficients are observed for compositions having up to 50 mole % sodium niobate additions to potassium niobate. The activity diminishes with additional sodium niobate content and disappears beyond about 98 mole % additions. Dielectric constants are relatively low, varying from 450 to 125, depending on composition. A.-c. losses are high and d.-c. resistivities are ∼1012ohm-cm. The low dielectric constants and high coupling obtained in certain regions of the system make these materials promising for solid ultrasonic delay line use, especially where thin sectioned plates are required, as in high-frequency thickness extensional or thickness shear mode transducers.

Dependence of Mechanical Strength of Brittle Polycrystalline Specimens on Porosity and Grain Size

Abstract: An empirical equation form, approximating the apparent dependence of the strength of brittle polycrystalline specimens on the combined effect of porosity and grain size, has been developed from concepts and data in the literature. The form is shown to be applicable to data for thoria and chromium carbide specimens.

The KaoIinite-Mullite Reaction Series: I, A Survey of Outstanding Problems

Abstract: This reaction series has been a subject of extensive investigation since the work of Le Chatelier in 1887. Nevertheless, major problems have remained concerned with the nature of metakaolin, the manner in which it transforms into a spinel-type phase and mullite, and the relation of this spinel-type phase to mullite. The present survey brings these problems into sharp focus and provides the necessary historical background fora new approach to their solution.

Young's Modulus of Various Refractory Materials as a Function of Temperature

Abstract: Young's modulus as a function of temperature was determined by a dynamic method for single-crystal sapphire and ruby and for polycrystalline aluminum oxides ma~esium oxide, thorium oxides mullites spinel, stabilized zirconium oxides silicon carbides and nickelbonded titanium carbide. For the single crystals, Young's modulus was found to decrease linearly with increasing temperature from 100 C to the highest temperature of measurement. For all the polycrystalline materials, except silicon carbide, stabilized zirconium oxides and spinel, Young's modulus was found to decrease approximately linearly with increasing temperature until some temperature range characteristic of the material was reached in which Young's modulus decreased very rapidly and in a nonlinear manner with increasing temperature. This rapid decrease at high temperature is attributed to grain-boundary slip. Stabilized zirconium oxide and spinel were found to have the same rapid decrease in Young's modulus at high temperatures but they also had a decidedly nonlinear temperature dependence at low temperature. (auth)

Electromechanical Properties of Lead Titanate Zirconate Ceramics Modified with Certain Three‐or Five‐Valent Additions

Abstract: When small amounts of lanthana, neodymia, niobia, or tantala are added to lead titanate zirconate compositions near the morphotropic (rhombohedral-tetragonal) phase boundary, the modified ceramics show piezoelectric planar coupling coefficients of about 0.50 and substantially increased dielectric constants of up to 1545. The mechanical Q factor is decreased to about 70, and there is a marked increase in volume resistivity at temperatures up to 500°C. Other effects noted are better shelf-aging characteristics, improved stability with respect to temperature dependence of resonant frequency, and a lowering of the Curie temperature. Except with neodymia, a definite refinement of the grain structure is also observed. By similar modification of Pb0.95Sr0.05(Zr.0.54 Ti0.45)O3 dielectric constants up to 1792 are obtained along with planar couplings of at least 0.49.

Oxygen Ion Mobility in Cubic Zr0.85Ca0.15O1.85

Abstract: Oxygen ion mobility in the cubic fluorite-structure phase having the composition Zr0.85Ca0.15O1.85 has been directly determined by exchange measurements employing the stable isotope O18 and mass spectrometer analysis. Measurements of electrical conductivity have been carried out as a function of both temperature and oxygen pressure. Results indicate that the conductivity is wholly ionic with a transfer number near unity for the oxygen ion.

Phase Equilibria in the System CaO‐Iron Oxide‐SiO2, in Air

Abstract: Phase equilibrium relations in the liquidus temperature region of the system CaO-iron oxide-SiO2 in air were determined, using the quenching technique. The results are illustrated by means of a projection of the liquidus surface onto the composition triangle CaO-Fe2O3-SiO2. A supplementary diagram is presented in order to indicate true compositions of liquids at liquidus temperatures. Data obtained in the present investigation are combined with literature data to construct diagrams showing stability relations among crystalline phases at subsolidus temperatures. A diagram is also presented to show phase relations along the join CaO SiO2-2CaO Fe2O3.

Surface Tension of Some Liquid Oxides and Their Temperature Coefficients

Abstract: Surface-tension data are reported for liquid Al2 O3, B2 O3, GeO2, P2 O5, and SiO2 Abnormal positive temperature coefficients for B2 O3, GeO2, and SiO2 are shown to be due mainly to changes in the liquid structure (dissociation) with temperature

Stable and Metastable Equilibria in the Systems Y2O2-Al2O3, and Gd2O3-Fe2O3

Abstract: The phase equilibrium relations in the systems Y2O3-Al2O3 and Gd2O3-Fe2O3 were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet-type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd2O3-Fe2O3 but only metastable in the system Y2O3-AI2O3. This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O3. A new 2:1 molar ratio cubic phase has been found in the system Y2O3-A12O3. Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination.

Oxidation of Silicon Carbide

Abstract: The rate of oxidation of silicon carbide was measured in an atmosphere of dry oxygen between 900° and 1600°C. The rate was studied by using a thermogravimetric apparatus and was found to be diffusion controlled. The products of oxidation were amorphous silica and cristobalite, depending on the temperature. The effect of surface area was determined, and a correlation between the various sizes was made with the aid of an equation derived on the assumptions that (1) the reaction was diffusion controlled, (2) the particles were essentially spherical, and (3) the interfacial area was constantly changing. The presence of water vapor in the gaseous atmosphere was found to be extremely critical.

Thermal Conductivity: XIV, Conductivity of Multicomponent Systems

Abstract: The thermal conductivity of a number of multi-component systems has been determined as a function of composition and temperature. These were silicate glasses, MgO-NiO, Al2O3-Cr2O3, UO2-UO2+x-ThO2, ZrO2-HfO2-CaO, MgO-Mg2-SiO4, MgO-BeO, Al2O3-mullite, MgO-MgAl2O4, Al2O3-ZrO2, Al2O3-glass, and Si-Sic. Analyses of these results indicate that a second component in solid solution markedly lowers the thermal conductivity. The second-component scattering mean free path is inversely proportional to concentration at low concentrations and independent of temperature at the temperatures studied (above room temperature). The conductivity of polyphase ceramics can be predicted if the conductivity of each phase, amount of each phase, and phase distribution (including pore phase) are known.

The KaoIinite‐MuIIite Reaction Series: Ill, The High‐Temperature Phases

Abstract: X-ray diffraction data for the high-temperature phases show that a spinel-type structure develops with marked orientation at about 925°C. This phase is considered to be an aluminum silicon spinel with vacant cation sites. Mullite is thought to be formed by the decomposition of the spinel. Silica is eliminated progressively as metakaolin transforms to the spinel phase and thence to mullite. The X-ray data show variation in the mullite parameters between 1200° and 1400°C.; at 1400°C., the composition probably approximates 3A12O3·2SiO2. The nature of the intermediate spinel-type phase is discussed in relation to the crystal chemistry of spinels.

Rare‐Earth Polymorphism and Phase Equilibria in Rare‐Earth Oxide‐Water Systems

Abstract: A high-water-pressure high-temperature study was conducted of the systems Y/sub 2/O/sub 3/-H/sub 2/O, Sm/sub 2/O/sub 3/-H/sub 2/O, N d/sub 2/O/sub 3/-H/sub 2/O, and La/sub 2/O/sub 3/-- H/sub 2/O. The trihydroxides La(OH)/sub 3/ , Nd(OH)a, an d this group. On the other hand, the oxyhydroxides YOOH, SmOOH, and NdOOH are isomorphous; LaOOH, if it exists, cannot be stable in the presence of water at room temperature. Pressure-temperature curves for the equilibrium between the trinydroxide, oxyhydroxide, and a new hydroxy compoud were determined for each of the three smaller ions. The reversibility of the transition between the A-, B-, and C-type rare earths was established, and new x-ray data on the B structure are listed. Ths relations between the polymorphs as a function of ionic size are changed from those given by Goldschmidt. The very marked difference in stability in air between the A- and C-type structures suggests the obvious desirability of converting rareearth oxide cermnics into the C structure with proper compositional variation and thermal treatment. (auth)

Thermal Contraction of β‐Eucryptite‐. (Li2O.Al2O3.2SiO2) by X‐Ray and Dilatometer Methods

Abstract: The axial thermal-expansion coefficients of β-eucryptite have been measured and correlated with the volume expansion of an aggregate measured dilatometrically. A mechanical analogy to the structure of β-eucryptite is suggested which may explain the pronounced thermal contraction in the c direction.

Nature of Adherence of Porcelain Enamels to Metals

Abstract: An investigation of the fundamentals of the adherence of porcelain enamels to metals indicated that good adherence is the result of metal-to-metal bonds between the atoms in the base metal and the proper metallic ions in the enamel. To accomplish this type of bond, the enamel must meet certain chemical and thermodynamic requirements: (1) The enamel at the interface must be saturated with an oxide of the metal and (2) this oxide must be one which, when in solution in the glass, will not be reduced by the metal. In the case of iron, the oxide is FeO. Many of the phenomena observed in commercial enameling were investigated and found to be related to adherence, but not essential for its development. An example is the precipitation of metallic particles in the enamel. Much of the complexity in commercial enameling arises from the limitations imposed by practical considerations. For example, because enamels usually are fired in air, the heavy scale developed during the early stages of firing must be removed before adherence can be developed. Likewise, as the conditions of the enamel-metal interface change rapidly during firing, “adherence-promoting oxides” are used to help maintain the necessary conditions for the time required in commercial enameling. Surface roughness, although not necessary for excellent adherence, was found to improve the apparent adherence when the bond between the enamel and the metal was relatively weak.

Aluminum Nitride, a Refractory for Aluminum to 2000°C.

Abstract: In commercial practice, where temperatures are low, a number of commercially available materials are satisfactory as a container for molten aluminum. None of these, however, resists attack by aluminum in the temperature range 1800 to 2000 deg C. An investigation was conducted to find a material completely inert to aluminum at these high temperatures. Aluminum nitride emerged as the material that was inert to aluminum aud showed satisfactory properties as a ceramic material. A methed of preparing high-purity A1N by striking a d-c arc between two high-purity aluminum electrodes in a nitrogen atmosphere is described. A refractory having excellent strength and low porosity was achieved by hydrostatic pressing and firing at high temperatures. The mechanical, electrical, thermal, and chemical properties of the refractory are discussed. (auth)

Formation of BaTiO3 from BaCO3 and TiO2 in Air and in CO2

Abstract: The formation of BaTiO3 from equimolar BaCO3 and TiO2 (rutile) mixtures was studied in air and in CO2. A small amount of BaTiO3 is formed first directly from BaCO3 and TiO2 at the surface of contact. From then on it is a diffusion-controlled reaction, and both BaTiO3 and Ba2TiO4 are produced, with Ba2TiO4 being formed in much larger amounts. In 1 atmosphere of CO2, the intermediate Ba2TiO4 was suppressed up to a temperature of about 1100°C. in agreement with thermodynamic calculations. Ba2TiO4 reacts fast with 1 atmosphere of CO2 below about 1100°C. to produce BaTiO3and BaCO3

Breaking Stress of Glass Determined from Dimensions of Fracture Mirrors

Abstract: Relations previously have been shown between the size of the fracture mirror and the nominal breaking stress. In the present paper, this relation is derived analytically involving one unknown quantity which must be determined experimentally. This quantity is δm√ρ where δm is the intrinsic strength of the glass and the effective radius of the crack tip. Tests on glass rods indicated that the breaking stress could be determined from the radius of the fracture mirror. For 80% of the specimens the values obtained were within 8% of those obtained from the test loads.

Electromechanical Properties of Lead Titanate Zirconate Ceramics with Lead Partially Replaced by Calcium or Strontium

Abstract: By partially substituting Sr for Pb in ceramic lead titanate zirconate compositions near the morpho-tropic boundary, dielectric constants greater than 1300 can be obtained along with planar couplings of about 0.50. Calcium also raises the dielectric constant, but the planar coupling level is considerably lower than that obtained with Sr. Other effects noted with Sr are a lowering of the Curie temperature, a shift in the morphotropic boundary toward the rhombohedral side, and a decrease in the distortion from cubic symmetry. In addition, Sr was found to be effective as a fluxing agent.

Strength and Static Fatigue of Abraded Glass Under Controlled Ambient Conditions: II, Effect of Various Abrasions and the Universal Fatigue Curve

Abstract: A controlled grit blast was found to be a reproducible method of producing standardized damage to a glass surface. The effects of grit size, blast pressure, and amount of grit on the strength of the resulting specimens are reported. Static fatigue curves (strength vs. load duration) were obtained for specimens immersed in room-temperature distilled water and in liquid nitrogen (77°K.) after the specimens had been subjected to various abrasion treatments. The low-temperature strength was independent of load duration, and for surface damage of simple geometry it was inversely proportional to the square root of the initial crack depth, consistent with the Griffith theory. Abrasions of different geometry produced differing static fatigue curves at room temperature, and in one case curves actually crossed. If, however, the strength values for each abrasion were divided by the low-temperature strength for that abrasion and plotted vs. a reduced time coordinate, all the data could be fitted to a single “universal fatigue curve.” This analysis led to a clear distinction between “linear” and “point” flaws, the former being flaws (such as emery scratches) which have an extension in a direction perpendicular to the applied stress and the latter being of a more localized character. Linear flaws fatigue more rapidly than point flaws by a factor of fifty and for each type of damage the fatigue rate is inversely proportional to the exponential of the initial flaw depth. A detailed analysis of the data in terms of several static fatigue theories from the literature shows that none of them provides a complete and adequate explanation of these results.

Phase Equilibria in the Ferrite Region of the System Fe—Co—O

Abstract: Phase relations in the system Fe-Co-O have been determined between 1000° and 1300°C in oxygen pressures of 0.01 to 20 atm. Only the portion of the system between CoFe2O4 and Co3O4 was considered in order to determine those conditions under which a complete series of solid solutions could be prepared. No field width on either side of oxygen stoichiometry was detected for solid solutions between CoFe2O4 and Co3O4.

Thermal Expansion of Al2O3, BeO, MgO, B4C, SiC, and TiC Above 1000°C.

Abstract: Thermal-expansion data on Al2O3, BeO, MgO, B4C, SiC, and TiC were obtained to the temperatures where permanent deformation begins, due to sintering or other causes. The thermal expansion for these materials was found to be approximately linear over the measured temperature range. But as a linear extrapolation to room temperature was not possible, the coefficient of thermal expansion is not a constant over this temperature range. The results are compared with the latest published data for each material. The average coefficients of linear thermal expansion are given as follows: All BeO specimens which were heated to above 2050°C. had a very large expansion. Visual examination of the cooled specimens revealed that they had bent and cracked, the physical dimensions had enlarged, and the color had changed to a bright milky white. A brief discussion of the probable reasons for these changes is given. In the attempt to extend the expansion measurements, the melting point of BeO was obtained. A specimen of hot-pressed BeO melted at 2450°± 30°C., whereas a slip-cast BeO specimen melted at 2470°± 20° C.

The Systems BaO‐SrO‐TiO2, BaO‐CaO‐TiO2, and SrO‐CaO‐TiO2

Abstract: The solid phases formed at 1400°C. in air in the three-component systems BaO-SrO-TiO2, BaO-CaO-TiO2, and SrO-CaO-TiO2 are described. Besides solid solutions of components with known structures, some new ternary compounds have been studied. The dielectric constants and loss factors of a number of specimens are given. Crystallographic data of the compounds BaCaTiO4, Ba3Ca2Ti2O9, and Ca3Ti2O7 and of the solid solution series (Ba, Sr), TiO4 are presented. The preparation of the new compounds is described in detail.

Studies in Lithium Oxide Systems: V, Li2O‐Li20 B2O3

Abstract: Nine compositions containing 40 to 68% B2O3 were used to study the high-lithia portion of the system Li2O-B2O3 by quenching and differential thermal analysis methods. The compounds 3Li2O 2B2O3 and 3Li2O B2O3 melted incongruently at 700°± 6°C, and 715°± 15°C., respectively. The compound 2Li2O B2O3 is assumed to dissociate slightly below 650°± 15° C., although the data could also be interpreted as in-congruent melting. Below 600°± 6°C. it does dissociate to the 3:2 and 3:1 compounds. In this narrow temperature interval the 2:1 compound had an inversion at 618°± 6°C. Both forms of the 2:1 compound could be quenched to room temperature. X-ray diffraction data for the compounds are tabulated, and the complete phase diagram for the system Li2O-B2O3 is presented.

Thermal Chemistry of Colloidal Titanium Dioxide

Abstract: Chemical and physical analyses have shown that hydrous titanium dioxide, prepared from titanium sulfate solution, consists of flocculates of small anatase crystals. Water and sulfur trioxide appear to be present as adsorbed layers and in capillaries between and within the flocculates. During heating, loss of water occurs at about 150°C., loss of sulfur trioxide at about 650°C., crystallite and particle-size growth at about 600°C., and the transformation of anatase to rutile in the range 700° to 950°C., depending on the method of preparing the samples. The kinetics of the transformation were found to be first order, following an induction time which appeared to depend on a nucleation process. Activation energies were of the order of 100 kcal. per mole and frequency factors of the order of 1020 to 1022 hr.-1. Precision X-ray diffraction studies showed that the anatase lattice expanded slightly before the transformation and that the initial rutile formed was expanded.

Observations on Growth of BaTiO3, Crystals from KF Solutions

Abstract: The purpose of this study was to try to find out why BaTiO3 (essentially a cubic structure) can consistently be grown from KF solutions as large flat plates when a more equant morphology is expected. It is suggested that the size, shape, and perfection of the platelike “butterfly” twin crystals can be qualitatively related to the re-entrant angles which result from {111} twinning of crystals of {100} habit. The re-entrant angles form permanent steps for both the advancing crystal tip and for the growth layers of the wings. The amount of twinning is related to the cooling rate and to the presence of certain impurities. Thus it can be shown that increasing the cooling rate increases the number of {111} twins (essentially a hexagonal stacking fault); the extreme case is the metastable formation of hexagonal barium titanate by quenching. Evidence is presented for the stabilization of the Ti2O9 group of the hexagonal structure by substitution of ions of lower polarizing power.

Strength and Static Fatigue of Abraded Glass Under Controlled Ambient Conditions: I, General Concepts and Apparatus

Abstract: An extensive experimental study of the factors affecting the tensile strength and static fatigue of bulk glass has been conducted. To minimize the effects of past history of the specimens, all specimens were subjected to a controlled, reproducible surface abrasion. Time, temperature, and chemical environment were subject to control and systematic variation during the period from abrasion to test and during the strength test itself. Specimens consisted of flat laths tested in cross bending with the abraded spot in the center of the tension face. An electronically controlled electromagnetic tester permitted applications of pulse loads or constantly increasing loads with controllable durations from about 0.0025 second up to any desired value. The apparatus and methods for producing the abrasions, controlling the environment, and performing the tests are described in this paper. A brief review of the experimental background on the strength of glass is also presented as an introduction to the aims and concepts of this study.

Strength of Synthetic Single Crystal Sapphire and Ruby as a Function of Temperature and Orientation

Abstract: The modulus of rupture of sapphire single crystals was determined as a function of temperature for specimens with orientations favoring plastic deformation and for specimens with unfavorable orientations. From 600°C. to l000°C., the strength of both types increased with increasing temperature, but the increase was more pronounced for the former. Ruby specimens oriented favorably for plastic deformation also showed a large increase in strength. It is conjectured that the increase in strength results from stress relief by microscopic plastic deformation.